Control of emissions from internal combustion engines has long been known to be a serious and difficult practical problem. The three major emissions (pollutants) from automobile engines are hydrocarbons, carbon monoxide, and oxides of nitrogen. These three pollutants are formed in substantially different physical processes. Oxides of nitrogen are formed by the endothermic reaction of nitrogen and oxygen in the hot post-flame gases at relatively high temperatures; formation of oxides of nitrogen as a function of temperature and concentration trajectories of individual lumps of gases burning in the cylinder.
Carbon monoxide emissions come primarily from burning of excessively rich lumps of gases in the main charge; uniformly lean combustion therefore reduces carbon monoxide to acceptable values. The mechanisms by which carbon monoxide is formed in engines are well understood matters of first-order chemistry having to do with incomplete CO burnout in rich zones of the combustion gases.
The production and emission of unburned hydrocarbons is more difficult to understand. At the temperatures and pressures characteristic of combustion in an internal combustion engine and of the exhaust blowdown event in an internal combustion engine, unburned hydrocarbon molecules should be completely reacted, provided sufficient oxygen is present and uniformly mixed throughout the charge. The chemical kinetic rates of hydrocarbon oxidation at these temperatures are exceptionally fast. Nonetheless, unburned hydrocarbon emissions are a difficult practical problem. HC control has historically imposed constraints on the control strategies for NOx emissions.
It has been known for some years that the exhaust hydrocarbon concentrations from an engine are found predominantly in two lumps: one occurs just after exhaust valve opening and the other occurs towards the very end of the exhaust stroke. Strong experimental evidence suggests that these two lumps of hydrocarbon together incorporate more than 95 percent of the total unburned hydrocarbon emissions from the engine. Although the heterogeneous nature of the hydrocarbon exhaust process has been known for a number of years (at least since SAE Paper #720112, "Time Resolved Measurements of Hydrocarbon Mass Flow Rate In The Exhaust of a Spark Ignition Engine" by R. J. Tabazinski, J. B. Heywood and James C. Keck), and although the process of heterogeneity has been investigated in a number of engine laboratories, there is still controversy as to the exact mechanisms of unburned hydrocarbon production. However, it seems extremely clear from both data and fluid mechanical theory that a large fraction (roughly half of the unburned hydrocarbons) comes from unburned hydrocarbons which are physically adsorbed or absorbed on the cylinder wall and/or from the oil layer on the cylinder wall. It appears that both that these layers are swept into a roll-up vortex which forms on the top of the piston and starting at the cylinder wall. This roll-up vortex carries relatively cold lumps of fluid, rich in hydrocarbon, from these wall layers. Hydrocarbon in these fluid lumps is not burned in the cylinder and is therefore exhausted. Much of this unburned HC does not oxidize in the exhaust port and exhaust manifolds, and therefore produces substantial unburned hydrocarbon emissions.
The roll-up vortex is a direct consequence of the relative motion between the piston and the slower moving fluid elements (boundary layer) near the surface of the cylinder as a result of the well-known no-slip hydrodynamic boundary condition of fluid mechanics. Considering the flow from a frame of reference moving with the piston, this boundary layer flow approaches the piston at or near piston velocity, which is a sufficient velocity to produce a significant intertial flow of the boundary layer fluid in towards the center of the piston face. This inwardly flowing fluid, under the conditions characteristic of piston engines currently made, creates a coherent roll-up vortex; this vortex has a toroidal shape with the major diameter of the order of or smaller than the cylinder diameter and a minor diameter of the order of 25 percent or less of the cylinder diameter. As the piston moves upward, the vortex moves radially inward, and the minor diameter of the roll-up toroidal vortex increases as more fluid layer is rolled into the vortex. The roll-up vortex is a relatively large flow structure which, in interaction with the cylinder combustion chamber geometry, makes exhaust of at least a significant part of the HC from the cylinder wall layer unavoidable in typical engine geometries. It is important to realize that the internal flow structure of the roll-up vortex is exceptionally coherent. The vortex is not well mixed, and is heterogeneous in temperature and local concentrations of combustables. As a result, the hydrocarbons in the roll-up vortex do not oxidize (burn). The roll-up vortex therefore systematically convects unburned hydrocarbon to a place where it will be exhausted, creating hydrocarbon emissions. It is important to note that the internal structure of the roll-up vortex appears laminar, and has no significant evidence of turbulent diffusion with a normally shaped piston face. Turbulent diffusion would be needed to mix the gases and cause oxidation of hydrocarbons before exhaust has reached the cylinder. Visualization of the roll-up vortex has been known in literature at least since the SAE paper #720112 by R. J. Tabazinski, J. B. Heywood and James C. Keck.
The roll-up vortex flow structure is exceptionally coherent, and the coherence of this flow structure is remarkably independent of the more obvious typical changes in the piston face geometry with the consequence that the roll-up vortex fluid mechanics is rather uniform over the entire population of current engines.
In view of the exceptionally high kinetic oxidation rates of hydrocarbons in oxygenating environments at temperatures characteristic of the exhaust process in an engine, it is highly desirable to disrupt the coherent laminar vortex structure of the conventional roll-up vortex and create a flow field with substantial turbulence in order to achieve dispersion to mix, heat and burn the unburned hydrocarbon from the oil and/or absorbed gases in the wall layers. Moreover, it is desirable to disrupt the flow geometry of a typical roll-up vortex into a flow geometry less likely to be exhausted near top dead center of piston motion. It is the purpose of the present invention to provide simple and manufacturable piston face geometries which do disrupt the roll-up vortex, thereby controlling hydrocarbon emissions.
A number of piston face geometries have been developed by the inventors which do significantly disrupt the conventional roll-up vortex in a reciprocating piston engine. The inventors had the idea of disrupting the roll-up vortex for mixing; they applied general fluid mechanical knowledge and detailed knowledge of vortex and boundary layer laminar and turbulent fluid mechanics. Using this knowledge the inventors developed an empirical testing program employing flow visualization which produced the modified piston geometries of the current invention. In each of the piston geometries shown, the inertial flow from the cylinder boundary layer is disrupted in such a fashion that the usual roll-up vortex does not form, but is instead replaced by a multiplicity of smaller, much more turbulent, and less stable flow structures which have much increased likelihood of dispersion and mixing.
As we will also show below, these same modified piston geometries have other potential benefits for design control and improvement of in-cylinder combustion processes in automobile engines. In particular, the modified geometries provide a powerful process, not previously exploited, for controlling both homogeneity and turbulence in intensity of the in-cylinder combustion charge at spark-firing time. Homogeneity of charge is critical in order to avoid the occurrence of individual lumps of fluid that are either rich (and therefore produce hydrocarbon emissions) or only slightly lean (thereby producing oxides of nitrogen). Turbulence intensity in the combustion charge at ignition time is also important in order to create rapid and complete combustion. The speed of combustion is linearly proportional to turbulence intensity (when other variables are held constant) as shown in the SAE Paper #760160, "Effects of Turbulence on Spark-Ignition Engine Combustion," by David Lancaster.